For example, in an automobile utilizing an internal combustion engine, fuel supply and ignition timing have been controlled for optimally controlling combustion against misfire, abnormal combustion, etc., by detecting the combustion pressure in a combustion chamber. Generally, a combustion pressure sensor utilizing an element with a piezoelectric oxide material exhibiting a piezoelectric effect (generating an electrical charge as a result of polarization occurring depending on an applied force (pressure)) has been used for detecting the combustion pressure in the combustion chamber.
In the piezoelectric element of a combustion pressure sensor, a single crystal of a piezoelectric oxide material is used. Conventionally, quartz has been used as a piezoelectric oxide material, langasite (LGS: La3Ga5SiO14) having a higher piezoelectric constant than that of quartz has received attention since the early 1990s, and research has been conducted with a particular emphasis on langanite (LGN: La3Ga5.5Nb0.5O14) having the same structure as that of langasite. Langatate (LTG: La3Ta0.5Ga5.5O14) has received attention due to having a small variation of a piezoelectric constant with temperature and a high insulating property, and LTG is currently widely used. Also, LTGA (La3Ta0.5Ga5.5-xAlxO14 (x=around 0.2 in most circumstances)) with a part of Ga of LTG substituted by Al has been known for 10 years or longer. LTGA has recently been shown to have a higher insulation resistance than that of LTG (Patent Literature 1) and has received attention.
Production of a single LTGA crystal has been performed by, e.g., a method (Czochralski method (CZ method)) of obtaining a single crystal by melting a sintered LTGA body (polycrystalline material) made by calcining a mixture of La2O3, Ta2O5, Ga2O3, and Al2O3 of a starting material weighed to give an objective stoichiometric single-crystal composition, immersing a seed crystal in a melt, and slowly pulling up the seed crystal.
As for a single LTG crystal as a ternary crystal, the congruent melting composition of a melt has been reported for obtaining a good single crystal with a few inclusions and cracks, and it is known to change the mixing ratio of lanthanum oxide, tantalum oxide, and gallium oxide of a starting material in the vicinity of a stoichiometric composition when a single crystal of a piezoelectric oxide material is produced from the melt (for example, see Patent Literature 2).
Among important characteristics necessary for a piezoelectric oxide material, an insulation resistivity has been known to be decreased by increasing temperature and to be decreased with a lapse of time. For example, Non Patent Literature 1 describes that, when a single LTG crystal grown in an oxygen atmosphere or an inert gas atmosphere is exposed to 550° C., insulation resistivity is decreased to around one-half of an initial value after 600 hours.